Phyllosilicates are composed of an irregular stack of elementary lamellae of crystalline structure, the number of which varies from several units to several tens of units. Among the phyllosilicates (lamellar silicates), the group comprising especially talc, mica and montmorillonite is characterized by the fact that each elementary lamella is constituted by the association of two tetrahedral layers situated on either side of an octahedral layer. This group corresponds to the 2:1 phyllosilicates, which include especially the smectites. In view of their structure, the 2:1 phyllosilicates are also described as being of the T.O.T. (tetrahedron-octahedron-tetrahedron) type.
The octahedral layer of the 2:1 phyllosilicates is formed of two planes of O2− and OH− ions (in the molar proportion O2−/OH− of 2/1). On either side of this middle layer there are two-dimensional lattices of tetrahedrons, of which one of the vertices is occupied by an oxygen of the octahedral layer, while the other three are occupied by substantially coplanar oxygens. In natural smectites, for example, the tetrahedral sites are generally occupied by Si4+ or Al3+ ions, and the octahedral sites are in mosts cases occupied by Mg2+, Fe2+, Al3+ and/or Fe3+ cations. A small proportion of the octahedral and/or tetrahedral sites of the smectites is not occupied and is responsible for the cation deficit of the crystalline lattice forming the elementary lamellae.
Some phyllosilicates, such as smectites, are also characterized by the presence, between the elementary lamellae, of interfoliar spaces which contain water and cations and which form a swelling phase of the mineral. The smectites are therefore described as being of the swelling T.O.T. type. In natural smectites, these interfoliar cations are generally Mg2+, Ca2+ and/or Na+ ions.
Phyllosilicate mineral particles, such as talc, are used in the form of fine particles in many industrial sectors, for example: rubber, thermoplastics, paper, paints, pharmacy, cosmetics or also phytosanitary products. They are used as an inert filler (for their chemical stability or for the dilution of expensive active compounds) or as functional fillers (for example to enhance the mechanical properties of some materials).
WO 2008/009800 proposes a method for preparing 2:1 phyllogermanosilicate-swelling phyllogermanosilicate (T.O.T.-swelling T.O.T.) interlayer compositions which can be used especially as a precursor for the preparation of talcose compositions, in particular synthetic talc. These T.O.T.-swelling T.O.T. interlayer compositions are synthesized by precipitation of a gel containing silicon and/or germanium and metal followed by a hydrothermal treatment of the gel at a temperature of from 150° to 300° C. The parameters used for carrying out the method for preparing these compositions allow some of the structural characteristics of the synthesized mineral particles to be controlled, in particular the crystallinity and the swelling mineral phase/non-swelling mineral phase ratio.
The synthesized T.O.T.-swelling T.O.T. interlayer particles described in WO 2008/009800 can have different colors depending on the metal salt used in the starting gel containing silicon and/or germanium and metal.
However, obtaining T.O.T.-swelling T.O.T. interlayer particles according to WO 2008/009800 requires that the protocol for synthesizing these particles be carried out in its entirety for each color desired, and that the synthesis parameters be changed for each product of a different color that is desired. Furthermore, once the T.O.T.-swelling T.O.T. interlayer particles have been synthesized, their color cannot be modified, the color being determined by the choice of reagents used in the first step of precipitation of the gel containing silicon and/or germanium and metal. Moreover, the choice of colors which can thus be obtained according to WO 2008/009800 remains limited, it being limited by the starting divalent metal salts which can be used.
In addition, although some natural talcs (steatites) exhibit varied colors such as pink, grey or green when they are in the form of blocks as obtained directly after extraction, grinding thereof into fine particles irretrievably leads to the obtaining of powders which are white to greyish in color, the color of the natural talc being due solely to the particular arrangement of the talc particles relative to one another. The grinding of such natural talcs therefore leads irretrievably to a loss of the possibly pink or greenish original natural color of the talc.